1. Field of the Invention
The present invention relates to a method for electrically connecting two wafers, and more particularly, to a method for electrically connecting wafers using a butting contact structure, which physically bonds two wafers according to an oxide-to-oxide bonding method and then electrically connects the two wafers using a butting contact structure.
2. Description of the Related Art
Recently, the process for reducing the physical size of an integrated circuit by reducing the sizes of devices on a wafer has reached the limit. In order to overcome such a limit, a method of vertically stacking two or more wafers has been proposed.
FIG. 1 is a cross-sectional view of a conventional semiconductor device fabricated through a metal-to-metal bonding method.
For convenience of description, suppose that the semiconductor device of FIG. 1 is a part of an image sensor. The semiconductor device includes an upper wafer 110 and a lower wafer 120. The upper wafer 110 includes a photodiode PD to generate charges corresponding to the amount of light incident from the top and a floating diffusion FD to temporarily store the charges. The lower wafer 120 includes elements constituting the image sensor excluding the photodiode PD.
Referring to FIG. 1, two wafers each having different elements implemented therein are bonded to each other. In FIG. 1, A, B, D, and D represents regions in which a metal as a conductive material or polycrystalline silicon is formed. The regions A and C formed in the upper wafer 110 must be electrically connected to the regions B and D formed in the lower wafer 120, respectively.
When a semiconductor fabrication process is completed, the top surface of a wafer is covered by an insulating material. Thus, when the top surfaces of two wafers are physically bonded to each other, the two wafers are electrically insulated from each other. For this reason, in order to electrically connect two wafers, conductive materials must be formed on the bonded portions of the two wafers. The conductive materials must be physically bonded to each other.
Referring to FIG. 1, a conductive material such as copper (Cu) may be formed on the surfaces of the regions A and C formed in the upper wafer 110 and the surfaces of the regions B and D formed in the lower wafer 120.
According to the above-described conventional metal-to-metal bonding method, the two wafers 110 and 120 are electrically connected to each other through the conductive layers formed on the surfaces of the regions A to D of the upper and lower wafers 110 and 120. Thus, the thickness of the wafers is inevitably increased. With the increase in thickness of the wafers, the lengths of wirings for electrical connection between elements formed in the two wafers 110 and 120 are increased. Thus, a power loss caused by resistance of the wirings cannot be ignored.
Furthermore, while each of the wafers is fabricated, the process of forming the conductive layer for electrical conduction must be separately performed during the bonding process. Thus, the time required for performing the process inevitably increases.